Radioactive batteries



' June 18, 1963 PPAP RT 3,094,634

RADIOACTIVE BATTERIES Filed June 30, 1955 592 Jmz'r/m/ [fad/V44 pa a/om l war T I lgww fPT/Ul? e fl 567/70 [11/5/16 Y INVENTOR.

fall Mia Haiti ATTORNEY United States Patent Ofi ice 3,094,634 Patented June 18, 1963 This invention relates to improved means for converting the energy of nuclear radiations into useful electrical energy. Particularly the invention relates to irradiating a semiconductive device with nuclear emissions to provide useful electrical power which is available at the output terminals of the device. The electric power thus provided may be utilized to supply voltage and cur-rent to a load circuit.

An object of the instant invention is to provide improved means for utilizing the energy of nuclear radiations.

Another object of the invention is to provide improved means for converting the energy of nuclear radiations into useful electrical energy.

Another object of the invention is to provide improved means for converting the energy of either charged or nuetral high energy radiations into useful electrical energy. 1

A further object of the invention is to provide improved voltage :and/ or current sources which employ radioactive isotopes.

A further object of the invention is to provide more efficient means for converting the energy of nuclear reactions into electrical energy.

A further object of the invention is to provide an improved radioactive battery having an extremely long life.

A still further object of the invention is to provide an improved radioactive battery which employs semiconductors.

A still further object of the invention is to provide a radioactive battery which is especially suitable for powering transistor circuits.

The foregoing objects and advantages are provided in accordance with the invention by disposing either a junction or a point-contact semi-conductor device in the path of radiations emitted by a radioactive source. The radiations penetrate the device to liberate charge carriers therein (electrons and holes) which flow within the device to provide a potential at its output terminals. The output energy may be utilized to supply current to a load circuit. Various embodiments of the invention are disclosed in which desired values of voltage and current may be achieved.

The invention will be described in detail with reference to the accompanying drawing in which:

FIGURE 1 is a schematic diagram of a radioactive battery, according to the invention;

FIGURE 2 is an energy level diagram which is helpful in describing the theory of operation of the device of FIGURE 1;

FIGURE 3 is a schematic diagram of an embodiment of the invention in which a point-contact semiconductive device is employed;

FIGURE 4 is a partially schematic view of a radioactive battery fabricated in accordance with the invention and having a plurality of semiconductive units connected in parallel for supplying current to a load circuit;

FEGURE 5 is a perspective view showing the manner in which ohmic connections may be made to the units of FIGURE 4;

' FIGURE 6 is a view including a schematic circuit diagram of a novel radioactive battery in which a plurality of junction-type semiconductive devices are connected in series;

FIGURES 7 and 8 are partially schematic views of further embodiments of the invention; and

\ FIGURE 9 is a perspective view of a still further embodiment of the invention in which a novel radioactive 'battery employs a semiconductive device having a plurality of grown junctions.

Similar reference characters are applied to similar elements throughout the drawing.

Referring to FIGURE 1, a novel radioactive battery includes a cold source 11 of high energy nuclear radiations. The term cold" is definitive of a radioactive source and is used herein in contradistinction to thermionic. The sources 11 may comprise one or a combination of materials which emit charged particle radiations and/or neutral radiations. Such emitters may include, for example, polonium and uranium (both emitters of positively charged alpha particles), strontium or tritium (emitters of negatively charged beta particles), cobalt (an emitter of neutral gamma rays), and numerous other radioactive isotopes.

The semiconductive device 13 to be irradiated comprises a p-n junction device either of the grown junction type or of the alloy type. A method of fabricating a grown type of junction having adjacent P and N regions as shown herein is disclosed in a copending application of G. K. Teal, Serial No. 168,184, filed June 15, l950, now US Patent 2,727,840. If the device is of the alloy type it may be fabricated, for example, as described in copending application Serial No. 294,741 filed June 20, 1952, assigned to the same 'assignee as the instant application, now US. Patent 2,894,862. With n-type germanium chosen for one region of the device, the material alloyed therewith to provide the adjacent p-type region may be indium, boron, or gallium. If p-type germanium or ptype silicon is used for one of the regions, the alloy material may be, for example, lead, antimony or goldantimony, respectively. Point-contact semiconductive devices also may be utilized as a part of the radioactive battery herein disclosed and claimed.

It may be assumed that the area and thickness of the semi-conductor device 13 are sufficient to absorb substantially all the radiations emitted by the source 11. For example, with a strontium source a germanium device having a thickness of the order of fifty mils is adequate. The thickness of a similarly irradiated silicon device is of the order of a hundred mils.

The junction device 13 is positioned in the path of the high energy radioactive emissions so that as much as possible of the incident radiations is absorbed in the junction region 15. The radiations emitted by the cold source 11 interact with the valence bonds in the solid semi-conductor device '13 (when a valence bond type of crystal as germanium or silicon is employed), causing charge carriers (electrons and holes) to be liberated within the solid. In the energy level diagram of FIGURE 2, the liberation of these charge carriers corresponds to raising electrons from the filled band 23 to the conduction band 25 thereby leaving behind holes in the filled band. With the incoming radiation having a minimum quantum energy which is equal to or greater than the energy gap of the empty or forbidden region, both electrons and holes are produced within the solid device 13. These charge carriers are available to take part in a current conduction process. The energy gaps for germanium and silicon, for example, are of the order of 0.72 electron volt and 1.l2 electron volts, respectively.

An electrostatic potential barrier exists in the junction region between the p and n sections of the device. Under the influence of this electrostatic potential the liberated charge carriers flow across the junction in one direction only. In the diagram electrons may be said to flow down the slope of curve 25 and holes flow up the slope of curve 23. Substantially all the charge carriers which get into the junction region may be expected to be collected and contribute to the terminal voltage and the output current of the device. Some of these carriers are produced in the junction region. Other charge carriers are produced outside the junction region 15 and initially are subjected to no electrostatic potential. However, if these carriers have sufiiciently great lifetimes and diffusion lengths and do not recombine with oppositely charged carriers, they also enter the junction region (solely by the diifusion process) and enhance the output current. The output current flows through the load circuit 21. The

circuit 21 may be connected to the irradiated semiconductive device 13 by ohmic connections 17 and 19, for example, solder, which provides contact to the p and n sections of the device, respectively.

The radioactive emitter material may be coated on one or more surfaces of the semi-conductor device to provide physical support for the emitter and to insure most efiicient use of the emission.

The following table comprises pertinent data for the radioactive battery of FIG. 1 when a 50 millicurie strontium emission source is employed.

where e is the charge of an electron, R is the rate of generation of charge carriers, L is the carrier diffusion length, m is a current multiplication factor, and I is the equivalent current of the radioactive source 11.

KT 1,, V e 111 where I is the junction reverse saturation current, T is absolute temperature in degrees Kelvin, and K is Boltzmanns constant.

The structure hereinbefore described affords a primary source of electrical energy which has numerous advantages. The battery size may be extremely small, of the order of a fraction of a cubic centimeter. The unit is a selg contained primary source in the sense that the electrical energy available at its output terminals is derived solely from the energy of radioactive emissions. No thermionic cathodes or external electrical inputs are required. The battery is rugged from a physical standpoint and is not afliected by vibration or mechanical shock. The nuclear-lto-electrical energy conversion efficiency is quite good as is evidenced by typical current multiplication factors listed in the above table. The battery impedance is appreciably lower than presently known primary radioactive energy sources and is particularly adapted for powering transistor and other circuits which require low voltages and currents. Additionally, the useful life of the energy source is extremely long. For example, if cobalt is used the source may last for more than five years while if the battery employs strontium it may last as long as twenty-five years.

FIGURE 3 shows an embodiment of the invention in which a point-contact semi-conductor device 24 rather than a junction rtype semi-conductor device is employed. The theory of operation is much the same as described above. The incoming nuclear emissions create charge carriers in the semi-conductive portion 26 of the device. The charge carriers created in the junction region surrounding the point-contact electrode 27 (and charge carriers which difiuse into this region) flow undirectionaliy across the junction under the influence of an electrostatic potential barrier. This conduction process results in a voltage being developed at the output terminals of the semiconductive device is energized by emissions from the radioactive source 11 and provides a portion of the output current of the device. Spacers 29 such as polystyrene or mica separate adjacent semiconductive units and prevent them from effectively short-circuiting each other. if germanium units are stacked in the manner described above and a strontium emission source is used, three or four junction units each having thicknesses from ten to fifteen mils are sufficient to almost completely absorb the radiations. If silicon units are used, the order of six units absorb the incident radiations. However, a greater number of thinner units may be used if desired. One advantage of the arrangement of FIG. 4 is that the high energy radiations penetrate the units in a direction transverse with respect to the junction region. Since some charge carriers are produced outside the junction region, and the units are thin, these carriers have shorter distances through which to travel to get into the junction region. This reduces the number of recombinations of electrons and holes which may occur and enhance the output current. This particular arrangement is desirable when using semiconductive materials in which the charge carriers have short diffusion lengths.

FIGURE 5 shows a convenient means by which the ohmic connections 17 and 1-9 of FIGURE 4 may be made. A pair of tabs 17 and 19 is afiixed to each of the spaced semiconductive units by any desired means such as a low melting point solder. One of the tabs 17 is connected to the p-type conductivity material while the other tab 19 is connected to the n-type conductivity material. The tabs 17 and 19 may be, for example, nickel and may be connected to the p and 11 type materials at any convenient portion of their surfaces.

FIGURE 6 shows another embodiment of the invention which is useful for producing higher voltages than those produced in the embodiments heretofore described. In the arrangement illustrated in FIGURE 6 the semiconductive units 1'3 are of the alloy type. As a typical example the body portion 31 of the device 13 may be n-type germanium and the p-type portion is the junction region shown by the dashed lines between the body portion 31 and a pellet of indium 33. However, other materials and other types of semiconductive devices such as those mentioned previously (i.e., grown junction and point-contact devices) also may be used. The semiconductive units 1'3 are stacked so that the incident nuclear radiation from the source 11 successively penetrates the units. The units are arranged so that the p-type pellet material of one semiconductive unit physically is butted against and contacts the n-type material of the next adjacent unit.

By so arranging the materials, ohmic contact is provided between adjacent units and the total output voltage of the battery is the sum of the voltages of the individual units. Electrical connection may be made to the stacked array for connecting the array to a load circuit 21 by a nickel tab 18 connected to the germanium material and a conductive lead 35 which makes contact with the indium pellet. The thickness and number of units which may be stacked is governed by substantially the same factors mentioned with respect to the structure of FIGURES 4 and 5 and also the terminal voltage desired from the stacked array. These factors include the energy and type of nuclear radiations emitted, the geometry cf the semiconductive :units, and the type of materials from which the units are fabricated.

In FIGURE 7 the radioactive source '11 is disposed between a pair of alloy type semiconductive units which are arranged in a back-to-back relation. The high energy radiations create charge carriers in each unit which flow across their rescpective junctions to provide a voltage at the output terminal of each unit. Ohmic connection between the n-region 31 of one unit and the n region 31 of the other unit is afforded via the radioactive emitter material 11 which preferably is supported by a conductive support member. The radioactive emitter is connected to one terminal of the load circuit 21. The pellets 33 which yield p-type conductivity to the body portion 31 of each unit are connected together and to the other terminal of the load circuit. The thickness of each unit 13 preferably is equivalent to the range of the radioactive emissions in the material. Advantages of the above described embodiments are an increase in output current by a factor of two', and more efficient utilization of the emissions produced by the cold source 11.

FIGURE 8 shows a further embodiment of the invention wherein the semiconductive unit 13 comprises an alloy type junction device having a body portion 31 into which two impurity pellets initially are alloyed and diffused. Electrostatic potential barriers 44 result from the alloying process and are created between the portion 31 and the pellets 33. If the body portion 31 is n-type germanium the impurity pellets diffused therein may be indium or one of the other materials heretofore mentioned. The impurity pellet which is alloyed and diffused into the portion of the semiconductive body which is to be spaced from but nearest the cold source 11 is then removed from the body portion 31. When the device thus fabricated is irradiated by the source 11, charge carriers flow in the direction of each electrostatic potential barrier. The fact that two barriers are provided is important since the number of recombinations of electrons and holes which tend to occur is reduced by a factor of the order of two. Since the recombinations are reduced, the output current supplied to the load circuit 21 increases by a corresponding amount. The purpose in removing the one impurity pellet adjacent the cold source 11 is for the purpose of eliminating material which would absorb the radioactive emissions without contributing to the output of the unit.

FIGURE 9 shows a still further embodiment of the invention in which a plurality of semiconductive devices 37 of the grown junction type are employed. Each device comprises a suitably shaped ingot or filament in which pand n-type conductivity regions occur alternately. The device 37 may be fabricated according to several methods. One of these methods is disclosed in copending application Serial No. 168,184, now US. Patent 2,727,840, cited previously. Briefly this method involves dipping a seed of germanium into a molten mass of germanium. The seed is withdrawn from the molten mass at a rate sufiicient to draw some of the molten mass therewith. As the seed is withdrawn the impurity balance in the melt is altered to effect a controlled variation in the conductivity, or an inversion in the conductivity,

of the melt and of the Withdrawn material. For example, if the melt is n-type initially, it may be converted to p-type by adding an acceptor material such as gallium. Reconversion to n-type is attainable by adding a donor material such as antimony.

The devices 37 each are similar to the series connected device shown in FIGURE 6 and are placed relatively close together. Interspersed between the units 37 is the radioactive emission material 11. Since the emitter 11 has a low ohmic resistance, insulating members 39 are disposed between the emitter 1 1 and the devices 37. In the present example the units are connected in parallel for supplying current at higher voltages to a load 21. However, if even higher voltages are desired it will be appreciated that the units may be connected in series with each other. Voltages of the order of volts may be 7 realized in this manner.

With the arrangement of FIGURE 9 (i.e., a plurality of grown junctions) it is essential that alternate junctions be made to have low ohmic resistance. Thus junctions 41 and 43 are destroyed by some such means as sandblasting or copper plating. If the junctions are not treated in this manner the current flowing across a given junction is approximately equal to and flows in a direction opposite to that of an adjacent junction. The net current produced by each unit 37 then would be almost zero.

What is claimed is:

11. A primary source of electrical energy comprising, a pair of semiconductive devices, each with a junction and regions of difierent conductivity on opposite sides of the junction, said devices being arranged with regions of the same conductivity adjacent each other, a cold source of high energy nuclear emissions positioned between said adjacent regions, and connection means coupled to said devices for deriving a load current.

2. A primary source of energy as claimed in claim 1 wherein said emission source is in contact with said adjacent regions and said devices are connected in parallel.

References Cited in the file of this patent UNITED STATES PATENTS 2,543,039 McK-ay Feb. 27, 1951 2,560,594 Pearson July 17, 1951 2,582,850 Rose Jan. 15, 1952 2,641,713 Shive June 9, 1953 2,650,311 Bray et a1. Aug. 25, 1953 2,661,431 Linder Dec. 1, 1953 2,670,441 McKay Feb. 23, 1954 2,719,253 Willardson et al Sept. 27, 1955 2,727,840 Teal Dec. 20, 1955 2,754,431 Johnson July 10, 1956 2,847,585 Christian Aug. 12, 1958 OTHER REFERENCES Physical Review, vol. 71, #2, pp. 129-30, Ian. 15, 1947. 

1. A PRIMARY SOURCE OF ELECTRICAL ENERGY COMPRISING, A PAIR OF SEMICONDUCTOR DEVICES, EACH WITH A JUNCTION AND REGIONS OF DIFFERENT CONDUCTIVITY ON OPPOSITE SIDES OF THE JUNCTION, SAID DEVICES BEING ARRANGED WITH REGIONS SOURCE OF HIGH ENERGY NUCLEAR EMISSONS POSITIONED BETWEEN SAID ADJACENT REGIONS, AND CONNECTION MEANS COUPLED TO SAID DEVICES FOR DERIVING A LOAD CURRENT. 